Thus far, an imaging device in which a single-photon avalanche diode (SPAD) is used as a photoelectric conversion element of a pixel has been used as an imaging device used for imaging or the measurement of the distance to a subject in a low-illuminance environment. The SPAD is a photodiode that performs photoelectric conversion in a state where a voltage exceeding the breakdown voltage is applied. Since a voltage exceeding the breakdown voltage has been applied, an electron avalanche derived from a carrier generated by photoelectric conversion occurs, and the SPAD enters a breakdown state. As a result, multiplication of carriers based on the photoelectric conversion is made, and an improvement in sensitivity in the imaging device is expected. However, since a relatively high voltage is applied to the SPAD, the SPAD requires a relatively large isolation region for being isolated from the surrounding circuits etc. Further, an electrode of the SPAD and wiring connected to the electrode are placed on a surface of the pixel. The region where the SPAD is formed corresponds to a light receiving surface in the surface of the pixel; therefore, by the isolation region etc. described above being placed, the ratio of the light receiving surface in the pixel surface, that is, the aperture ratio is reduced. Hence, photon detection efficiency is a relatively low value. Here, the detection efficiency is the ratio of the number of detected photons to the number of incident photons, and is a value showing characteristics of photon detection.
Thus, an imaging device in which light incident on an isolation region is guided to a SPAD and thereby detection efficiency is improved is used. For example, an imaging device in which a microlens is placed for each pixel and light is collected to a region of a SPAD that is formed in a central portion of the pixel is proposed (for example, see Patent Literature 1).